Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

Control of metal oxides' electronic conductivity through visual intercalation chemical reactions

Cation intercalation is an effective method to optimize the electronic structures of metal oxides, but tuning intercalation structure and conductivity by manipulating ion movement is difficult. Here, we report a visual topochemical synthesis strategy to control intercalation pathways and structures and realize the rapid synthesis of flexible conductive metal oxide films in one minute at room temperature. Using flexible TiO2 nanofiber films as the prototype, we design three charge-driven models to intercalate preset Li+-ions into the TiO2 lattice slowly (µm/s), rapidly (mm/s), or ultrafast (cm/s). The Li+-intercalation causes real-time color changes of the TiO2 films from white to blue and then black, corresponding to the structures of LixTiO2 and LixTiO2-δ, and the enhanced conductivity from 0 to 1 and 40 S/m. This work realizes large-scale and rapid synthesis of flexible TiO2 nanofiber films with tunable conductivity and is expected to extend the synthesis to other conductive metal oxide films.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy

Anatase TiO₂ is a promising material for Li-ion (Li⁺ ) batteries with fast charging capability. However, Li⁺ (de)intercalation dynamics in TiO₂ remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO₂ nanoparticle clusters. Directly probing active nanoparticles revealed that TiO₂ with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li⁺ storage in TiO₂ particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

Retarding the Shuttling Ions in the Electrochromic TiO2 with Extensive Crystallographic Imperfections

To understand the role of structure imperfections on the performance of electrochromic transition metal oxide (ETMO) is challenging for the design of efficient smart windows. Herein, we investigate the performance evolution with tunable crystallographic imperfections for rutile TiO₂ nanowire film (TNF). Structure imperfections, originating mainly from the copious oxygen deficiency, are apt to cumulatively retard the shuttling ions, resulting in the response rate for raw TNF being less than the half that of TNF annealed at 500 °C. We describe ion accommodation sites as a convolution of normal site and abnormal site, in which the normal site performs reversible coloration but the abnormal site contributes only to charge storage, which gives a rationale for the non-linear coloration and rate capability loss. These findings give a clear picture of the ion shuttling process, which is insightful for enhancing the electrochromic performance via structure reprogramming.

Electrochemically induced amorphous-to-rock-salt phase transformation in niobium oxide electrode for Li-ion batteries

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb₂O₅ electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li⁺. This electrode can reversibly cycle three lithiums per Nb₂O₅, corresponding to a capacity of 269 mAh g^-1 at 20 mA g^-1, and retains a capacity of 191 mAh g^-1 at a high rate of 1 A g^-1. It exhibits superb cycling stability with a capacity of 225 mAh g^-1 at 200 mA g^-1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials.

Wide-Spectrum Modulated Electrochromic Smart Windows Based on MnO2/PB Films

Inorganic materials have been extensively studied for visible electrochromism in the past few decades. However, the single inorganic electrochromic (EC) material commonly exhibits a single color change, leading to a narrow spectrum of modulation, which offsets or limits the maximally energy-saving ability. Here, we present a wide-spectrum modulated EC device designed by combining the complementary EC nanocomposite of manganese dioxide (MnO₂) and Prussian blue (PB) for enhanced energy savings. Porous MnO₂ nanostructures serve as host frameworks for the templated growth of PB, resulting in MnO₂/PB nanocomposites. The complementary optical modulation ranges of MnO₂ and PB enable a widen-spectrum modulation across the solar region with the development of the MnO₂/PB nanocomposite. The colored MnO₂/PB device exhibited an optical modulation of 32.1% in the wide solar spectrum range of 320-1100 nm and blocked 72.0% of the solar irradiance. Furthermore, fast switching responses (2.7 s for coloration and 2.1 s for bleaching) and a high coloration efficiency (83.1 cm²·C^-1) of the MnO₂/PB EC device are also achieved. The high EC performance of the MnO₂/PB nanocomposite device provides a new strategy for the design of high-performance energy-saving EC smart windows.